Laser power calibration and correction

ABSTRACT

A LIDAR system emits laser pulses, wherein each pulse is associated with a power level. A laser emitter is adjusted during operation of a LIDAR system using power profile data associated with the laser. The power profile data is obtained during a calibration procedure and includes information that associates charge duration for a laser power supply with the actual power output by laser. The power profiles can be used during operation of the LIDAR system. A laser pulse can be emitted, the reflected light from the pulse received and analyzed, and the power of the next pulse can be adjusted based on a lookup within the power profile for the laser. For instance, if the power returned from a pulse is too high (e.g., above some specified threshold), the power of the next pulse is reduced to a specific value based on the power profile.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/440,734, filed Dec. 30, 2016, which is incorporated herein byreference.

BACKGROUND

“LIDAR” refers to a technique for measuring distances of visiblesurfaces by emitting light and measuring properties of the reflectionsof the light. The term “LIDAR” is an acronym for “Light Detection andRanging” and is sometimes referred to as “laser scanning” or “3Dscanning.” In some cases, a LIDAR system includes multiple laseremitters and/or multiple light sensors. Alternatively, or in addition, aLIDAR system may physically move one or more lasers and/or sensors toscan over a scene while repeatedly taking measurements of differentsurface points.

Generally, the light emitter may comprise a laser that directs highlyfocused light in the direction of an object or surface. The light sensormay comprise a photodetector such as a photomultiplier or avalanchephotodiode (APD) that converts light intensity to a correspondingelectrical signal. Optical elements such as lenses may be used in thelight transmission and reception paths to focus light, depending on theparticular nature of the LIDAR system.

A LIDAR system has signal processing components that analyze reflectedlight signals to determine the distances to surfaces from which theemitted laser light has been reflected. For example, the system maymeasure the “flight time” of a light signal as it travels from thelaser, to the surface, and back to the light sensor. A distance is thencalculated based on the flight time and the known speed of light.

LIDAR systems can be used to inform guidance, navigation, and controlsystems such as may be used in autonomous vehicles. In systems such asthis, one or more LIDAR devices are configured to produce a surface mapindicating the 3D coordinates of visible surface points surrounding thevehicle. A guidance, navigation, and control system analyzes this datato identify obstacles, to perform obstacle avoidance, and to determine adesired path of travel. Developing and creating LIDAR systems that areboth accurate and have the desired resolution for a particularapplication can be costly and challenging.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Theuse of the same reference numbers in different figures indicates similaror identical components or features.

FIG. 1 illustrates an example of a laser power calibration andcorrection system.

FIG. 2 illustrates logical elements of an example LIDAR distancemeasurement system that may be used to perform distance or rangingmeasurements.

FIG. 3 shows an example process for adjusting the charging time of alaser light emitter to generate a light pulse at a specified powerlevel.

FIG. 4 shows an example process for generating a power profile for alaser light emitter.

FIGS. 5A and 5B illustrate an example configuration of a rotatablesensor assembly that may be used as part of a LIDAR sensor device orsystem.

FIG. 6 is a representational top view of an example light sensor thatmay be used in the LIDAR assembly of FIGS. 5A and 5B.

FIG. 7 is a representational top view of the example light sensor,illustrating an example packing arrangement.

FIG. 8 is a representational top view of an example laser light sourcethat may be used in the LIDAR assembly of FIGS. 5A and 5B.

FIG. 9 is a schematic view of an example electrical circuit that may beused in a measurement channel to generate a single laser pulse.

FIG. 10A is a schematic view of an example electrical circuit that maybe used in a measurement channel to generate a pair of laser pulses.

FIG. 10B is a schematic view of a trigger circuit that may be used in ameasurement channel to fire a laser emitter.

DETAILED DESCRIPTION

The following detailed description is directed to technologies for laserpower calibration and correction. Techniques are described herein forgenerating a power profile for a laser light source and then using thepower profile to adjust the power of emitted light pulses duringoperation. The power profile is accessed to determine a charging timethat will cause the light source to generate and emit a light pulse at aspecified power level. In contrast to slowly changing the power of anemitted light pulse, techniques described herein use a charge durationstored within one or more power profiles to generate the light pulse atthe desired power level. In this way, the power of each subsequentlyemitted light pulse can be adjusted to have the desired power level.

The power profile includes data that indicates the average power outputby the laser light source at different charging durations. In someconfigurations, instead of using a single power profile to represent thelasers within a LIDAR system, a separate power profile is generated foreach laser within the LIDAR system. For instance, when thirty eightlaser emitters are used within a LIDAR system, a power profile for eachlaser emitter is generated. In this way, differences between thephysical components utilized in each of the laser emitters of the LIDARsystem can be better accounted for as compared to using a representativepower profile for the lasers of the LIDAR system.

In some examples, the power profiles are determined during a calibrationcycle that can be performed before or after deploying the LIDAR system.During a calibration cycle, a laser emitter is aimed at a referencesurface. Generally, the reference surface is at a specified distancefrom the laser emitter and has known reflective properties. As discussedbriefly above, the power profile for each laser emitter includes theaverage power of light pulses emitted by the laser emitter usingdifferent charge times. The number of charge durations and correspondingpower values included within a power profile can change betweendifferent applications. For instance, some applications can include fivepairs of values whereas another application can have twenty or morepairs of values. Generally, the more data recorded within the powerprofile, the more finely the power output of the laser emitter can becontrolled within the LIDAR system. After charging the charge circuitfor the determined charge duration, a capacitive circuit drives thelaser emitter to produce an output light pulse. Thousands of differentlight pulses can be generated using each of the different chargedurations in order to obtain an accurate power of the laser at thespecific charge time.

After generating the power profiles, information from the power profilescan be used during operation of the LIDAR system. For example, a laserpulse can be generated, the reflected light from the pulse received andanalyzed, and the power of the next pulse can be adjusted based on alookup within the power profile for the laser. For instance, if thepower returned from a pulse is too high (e.g., above some specifiedthreshold), the power of the next pulse can be reduced to a specificvalue based on the power profile. Similarly, if the power returned froma pulse is too low (e.g., below some specified threshold), the power ofthe next pulse can be increased to a specific value based on the powerprofile. When the power returned by a pulse is too high or too low, therange data can be inaccurate. For example, an ADC used by the LIDARsystem may not be configured to accurately represent the reflectedpulse.

Instead of having to successively adjust a charge time of a laser toslowly converge toward a desired power level, the power profilespecifies the corrected charge duration for the desired power level forthe next laser pulse. As such, a successive laser light pulse can beemitted such that the power of the reflected light is within a desiredrange. Additionally, the power used by the LIDAR system is reduced sincethe LIDAR system does not always stay at the same power level, or slowlyconverge toward a specified power level.

According to some examples, the LIDAR system includes laser emitters,light sensors, analog to digital converters (ADCs) and power suppliesmounted in a chassis that rotates about a vertical rotational axis toscan horizontally across a scene. During a rotation of the chassis,laser light pulses are emitted at different vertical directions and atdifferent horizontal directions. The vertical angle of light emission isvaried by using lasers that are at different positions within thechassis. The horizontal angle of light emission varies with the rotationof the chassis. The apparatus has one or more lenses that define a fieldof view of a scene surrounding the apparatus. As the chassis rotates,the field of view moves or scans horizontally. More details are providedbelow with regard to FIGS. 1-10.

FIG. 1 illustrates an example of a laser power calibration andcorrection system. As shown, system 100 includes a ranging device 102that comprises one or more laser light source(s) 104, a charge circuit106, sensor(s) 108, analog-to-digital converter (ADC) 110, controller112, and data store 114 that stores power profile 116.

In the current example, the ranging device 102 is configured to generatethree-dimensional coordinates of surfaces that are visible from theperspective of the laser light source(s) 104. In some examples, theranging device 102 can be used by guidance, navigation, and controlsystems of autonomous vehicles such automobiles, aircraft, boats, etc.The ranging device can also be used in other applications that have aneed for real-time, multi-point, scanning distance measurements. Thelaser light source(s) can include one or more laser emitters, such asthe example ranging device illustrated in FIGS. 5-10.

The system 100 has a controller 112 that implements control and analysislogic for the ranging device 102. The controller 112 may be implementedin part by an FPGA (field-programmable gate array), a microprocessor, aDSP (digital signal processor), or a combination of one or more of theseand other control and processing elements, and may have associatedmemory for storing associated programs and data.

To initiate a single distance measurement, the controller 112 instructsthe charge circuit 106 to charge for a specified charge duration. Thesignal generated by the charge circuit is used by the laser light sourceto generate a light pulse. In some examples, the controller 112 causesthe charge circuit 106 to charge one or more capacitors for thespecified charge duration. After charging for the specified period oftime, the controller 112, or some other component can cause the laserlight source 104 to generate and emit a light pulse at a power levelthat is related to the charge time of the charge circuit 106.

As illustrated, for a single distance measurement, the laser lightsource 104 is controlled by the controller 112 to emit one or more laserlight pulses along an outward path. In the current example, the laserlight source 104 emits a first pulse 122A that hits object 118A.Assuming that the emitted laser light is reflected from the object 118A,the sensor 108 receives the reflected light and produces a return signalused by the ADC 110 to generate a digital representation of the signal.

The return signal is generally of the same shape as the light pulsegenerated by the laser light source 104, although it may differ to someextent as a result of noise, interference, cross-talk between differentemitter/sensor pairs, interfering signals from other LIDAR devices, andso forth. The return signal will also be delayed with respect to thelight pulse emitted by the laser light source 104 by an amountcorresponding to the round-trip propagation time of the emitted laserpulse. The ADC 110 receives and digitizes the return signal to produce adigitized return signal that is a stream of digital values indicatingthe magnitude of the return signal over time.

According to some configurations, the controller 112 adjusts the chargeduration of the charge circuit 106 during operation of the rangingdevice 102 in order to adjust for the different reflective properties ofdifferent objects that a laser light pulse may hit. For instance, object118A may not reflect as much light as object 118B. As such, the signalreceived by the sensor 108 in response to a light pulse reflecting offobject 118A may not be of sufficient strength to generate an accuratedistance to the object 118A. Similarly, if an object, such as object118B reflects too much light, the magnitude of the return signal may notbe correctly identified by the ADC 110. As a result, the accuracy of therange measurement can also be reduced. In order to obtain more accurateresults, in some examples, the controller 112 is configured to adjustthe power of a subsequent light pulse such that the power of the lightpulse is within a predetermined range. The power consumption and eyesafety of the LIDAR system is also improved since the power is reducedin some situations and in other situations the laser power is limited.In this way, the return signal generated from the light pulse reflectingoff of an object will fall within the predetermined range and theaccuracy of the LIDAR system is improved. In other configurations, thepower of the laser light source is limited by an eye safe power level.For example, the laser light source can be an American NationalStandards Institute (ANSI) level 1 laser such that the maximum powergenerated by the laser light source is still eye safe.

In order to further clarify, a non-limiting example will be presented.In the current example, the first light pulse 122A is identified by thecontroller 112 to fall below a desired power level (e.g., as set by anauthorized user/operator of the ranging device 102). In response todetermining that the power level is below the desired power level, thecontroller 112 instructs the charge circuit 106 to charge for a chargeduration that is associated with the desired power level. In contrast toslowly changing the charging time to reach the desired power level, thecontroller 112 accesses the power profile 116 within data store 114 todetermine the charge duration of the charge circuit 106 to produce alight pulse at the desired power level. In this way, the power of theemitted light pulse can often times be adjusted before the laser movesto another object within the environment. Generally, the power profile116 includes data that indicates the average power output by the laserat different charging times. In some examples, the power profile 116correlates capacitor charge energy with emitted laser intensity for eachof the different lasers in the LIDAR system.

According to some examples, average power values for different chargetimes for each light source 104 of a ranging device 102 is tested duringone or more calibration cycles of the ranging device 102. In someconfigurations, the power values of each light source 104 are tested atdifferent charge times. For example, the controller 112 may emit lightpulses using the same charge duration for a period of time and averagethe power of the light emitted by the light source. After recording thecharge duration and the average power within the power profile 116, thecontroller 112 can use a different charge time to obtain a differentpower value. The controller 112, or some other component, can performthis operation for many different charge durations (e.g., 1 μs, 2 μs . .. N μs).

As briefly discussed, instead of using a single power profile as arepresentative power profile for all of the laser light sources 104within a LIDAR system, a separate power profile can be generated andstored for each laser within the LIDAR system. For instance, when thirtyeight laser emitters are used (or some other number of laser emittersare used), a power profile for each laser emitter is generated. In thisway, differences between the physical components utilized in each of thelasers of the LIDAR system can be better accounted for as compared tousing a representative power profile for the lasers of the LIDAR system.For example, the charging time needed to generate a light pulse at adesired power level can vary based on the location of the laser emitterwithin the LIDAR system, the differences between capacitors, inductorsand/or other electronic components utilized to generate a pulse, and thelike. In some examples, the power profile is associated with theassociated charge circuit 106 and the associated light source 104.

During a calibration cycle, a laser light source 104 is aimed at areference surface. Generally, the reference surface is at a specifieddistance from the laser emitter and has known reflective properties. Asdiscussed briefly above, the power profile for each laser emitterincludes the average power of light pulses emitted by the laser emitterusing different charge times. The number of charge times andcorresponding power values included within a power profile 114 canchange between different applications. For instance, some applicationscan include five pairs of values whereas another application can havetwenty or more pairs of values. Generally, the more data recorded withinthe power profile 116, the more finely the power output of the laseremitter can be controlled. After charging the charge circuit 106 for thedetermined charge time, the stored charge is used to generate an outputlight pulse. Thousands of light pulses can be generated using each ofthe different charge times in order to obtain an accurate power of thelaser at the specific charge time.

Returning to the above example, after the controller 112 determines toadjust the power of a subsequent light pulse to a specified value (orwithin an acceptable range), the controller 112 accesses the powerprofile 116 associated with the light source 104 to determine the chargeduration. In some examples, the controller 112 identifies the powervalue associated with a midpoint of the digital representation of thereturn signal and determines the value of the charge duration associatedwith the midpoint. In this way, a reflected signal that is higher orlower than expected may still be within an acceptable range. Once thecharge duration for the desired power level is determined, thecontroller 112 charges the charge circuit 106 for the specified durationand then causes the subsequent light pulse 122B to be emitted. Forpurposes of illustration, the dashed lines 122A and 122E indicate alower power as compared to the solid lines 122B-122D.

The controller 112 can be configured to determine whether to adjust thepower for each emitted light pulse, for every Nth pulse, or use someother metric in determining when to adjust the power of the lightsource. In the current example, the controller 112 determines that thereturn signal is within range until receiving the reflected light from122D. For example, the object 118B may have a higher reflectivity ascompared to object 118A, and as such the controller 112 determines todecrease the power of the light pulse 122E.

FIG. 2 illustrates logical elements of a LIDAR distance measurementsystem 200 that may be used to perform distance or ranging measurements.While one measurement channel is illustrated, a LIDAR system can includemany different measurement channels.

A measurement channel includes one laser light source, such as laseremitter 104 and a corresponding sensor element 108. For a singledistance measurement, the laser emitter 104 is controlled to emit one ormore laser light pulses through the lens 208A along an outward path 202.The burst is reflected by a surface 204 of a scene, through the lens208B, and to the sensor element 108 along a return path 206.

The lens 208B is designed so that beams from laser emitters at differentphysical positions within the ranging device 102 are directed outwardlyat different angles. Specifically, the lens 208B is designed to directlight from the laser emitter 104 of a particular channel in acorresponding and unique direction. The lens 208A is designed so thatthe corresponding sensor element 108 of the channel receives reflectedlight from the same direction.

The system 200 has a controller 112 that implements control and analysislogic for multiple channels. To initiate a single distance measurementusing a single channel, the controller 112 generates a signal 210. Thesignal 210 is received by the charge circuit 106. In response toreceiving the signal 210, the charge circuit 106 provides signal 214 tocharge the capacitive driver 216 for a specified duration.

After charging for the specified duration, the capacitive driver 216provides an emitter drive signal 218. The emitter drive signal 218 isconnected to the laser emitter 104 to pulse the laser emitter 104 and toproduce a pulse of laser light.

Assuming that the emitted laser light is reflected from the surface 204,the sensor element 108 receives the reflected light and produces areturn signal 220. The return signal 220 is generally of the same shapeas the emitter drive signal 218, although it may differ to some extentas a result of noise, interference, cross-talk between differentemitter/sensor pairs, interfering signals from other LIDAR devices, andso forth. The return signal 220 will also be delayed with respect to theemitter drive signal 218 by an amount corresponding to the round-trippropagation time of the emitted laser burst.

The ADC 110 receives and digitizes the return signal 220 to produce adigitized return signal 224. The digitized return signal 224 is a streamof digital values indicating the magnitude of the return signal 220 overtime.

A cross-correlation component 226 receives the digitized return signal224 and performs a cross-correlation between the digitized return signal224 and a reference waveform 228, to produce a cross-correlation signal230. In some configurations, the function of the cross-correlationcomponent 226 may be performed by the controller 112. In other examples,other mechanisms can be used to perform pulse detection.

The reference waveform 228 represents the timing and the intensity ofthe light that is actually emitted by the laser emitter 104. In certainexamples, the reference waveform 228 may be obtained during acalibration cycle. For example, in some examples there may be areference surface at which the output of the laser emitter can be aimed.The reference surface may be at a known, relatively small distance fromthe lenses 208A and 208B. When the output of the laser emitter 104 isdirected toward the reference surface, the capacitive driver 216 drivesthe laser emitter 104 to produce an output burst. The sensor element 108and the ADC 110 are then used to capture a waveform corresponding to thelight reflected from the reference surface. This captured waveform maybe used as the reference waveform 228. The reference waveform 228 may becaptured uniquely for each channel, may be stored and used for multiplesubsequent measurements, and may be updated over time to account forthermal drift and/or other variables. In some examples, the referencewaveform 228 may be updated at least once per revolution of the chassis.

In other examples, one or more different sensors may be used to capturethe reference waveform 228 during one or more calibration emissions ofthe laser emitter 104. Furthermore, multiple readings may be performedand averaged to create the reference waveform 228.

The controller 112 receives the cross-correlation signal and detectsand/or analyzes the cross-correlation signal 230 and possibly one ormore other signals, such as the digitized signal 224. The controller candetermine the magnitude of the signal 230 as well as determine to findits highest peak, which indicates the phase difference or time shiftbetween the light pulses as emitted from the laser emitter 104 and asreceived at the sensor element 108. The controller 112 can alsodetermine if the power of the return signal is within an acceptablerange (i.e., not too high or low).

FIGS. 3 and 4 show example processes for laser power calibration andadjustment. The example processes will be described as being performedin an environment having one or more LIDAR measurement channels, whichare used to perform respective distance measurements. In the describedexamples, each measurement channel comprises a charging circuit 106powering a laser emitter 104 and a corresponding light sensor 108. Thelaser emitters and sensors may be arranged as described herein or invarious different ways. In the environment described herein, any of theactions described can be performed, controlled, or supervised at leastin part by the controller 112 referenced in FIGS. 1 and 2.

FIG. 3 shows an example process 300 for adjusting the charging time of alaser light emitter to generate a light pulse at a specified powerlevel. An action 302 comprises charging a charge circuit 106 for thepredetermined time. As described herein, one or more capacitors can bepart of a charge circuit 106 that is used by a light source 104 to emita laser light pulse. Generally, the longer the charge duration, the morepower for an emitted light pulse.

An action 304 comprises generating and emitting one or more light pulsesusing the stored charge. In some examples more than one light pulse canbe emitted. When the emitted burst include more than one light pulse,the pulses separated in time by a time interval having a duration.

An action 306 comprises sensing a reflected light pulse corresponding toan emitted light pulse. This action is performed by the sensor element108 of the channel corresponding to the laser emitter 104 from which theemitted light pulse originated.

An action 308 comprises determining a power associated with thereflected light pulse. In some configurations, the action 308 includesdigitizing a signal produced by the sensor element 108 to produce adigitized return light signal. The digitizing is performed by the ADC110 associated with the channel.

An action 310 comprises determining whether the power of the reflectedlight is within a specified range. As discussed above, the specifiedpower level can be a single value or a range of values. In someconfigurations, the specified power level is set to a value that is ator near the midpoint of a resolution of the ADC 110. Higher or lowervalues can be utilized. When the power is within the specified range,the process 300 flows to action 302 where the same charging time can beutilized for generating the next light pulse. When the power is notwithin the specified range, the process 300 flows to action 312.

An action 312 comprises accessing a power profile associated with alight source. As described above, the power profile 116 includes datathat indicates the average power associated with light pulses emitted bythe laser using different charging times. In some configurations, thepower profile 116 can be stored in a data store 114, or some othermemory.

An action 314 comprises identifying a charge duration for the laser thatresults in an emitted light pulse at the specified range. In someexamples, the controller 112 performs a look up operation that locatesthe specified power within the power profile 116 (or a value near thespecified power level) and identifies the associated charge duration.

An action 316 comprises setting the charge duration to the timeidentified from the power profile. The process 300 can then return toaction 302.

FIG. 4 shows an example process 400 for generating a power profile for alaser light emitter. An action 402 comprises charging the charge circuit106 of a laser light source 104 for a charge duration. As describedherein, one or more capacitors can be part of a charge circuit 106 thatis used by a light source 104 to emit a laser light pulse.

An action 404 comprises generating and emitting light pulses using thecharge circuit 106 charged to the charge duration. As described above,the laser light source 104 can be controlled by the controller 112 togenerate light pulses using the same charge duration for a specifiedperiod of time.

An action 406 senses the reflected light pulses. As described above, thesensor element 108 senses the reflected light pulses corresponding tothe emitted light pulses.

An action 408 comprises determining an average power associated with thereflected light pulses. In some configurations, the action 408 includesdigitizing, using the ADC 110, the signals produced by the sensorelement 108 to produce digitized return light signals. The average powerlevel associated with the pulses can be determined by dividing the totalpower by the number of pulses emitted by the laser light source 104.

An action 410 comprises storing the power value for the charge durationwithin a power profile associated with the laser light source 104. Asdescribed above, the power profile can include many different pairs ofcharge durations and average power values.

An action 412 comprises adjusting the charge duration. For example, thecharge duration can be incremented/decremented some set amount (e.g. +−5μs, 10 μs, . . . ). The process 400 can then return to action 402.

FIGS. 5A and 5B illustrate an example configuration of a rotatablesensor assembly 500 that may be used as part of a LIDAR sensor device orsystem.

The sensor assembly 500 comprises a chassis 502 that rotates about arotational axis 504. In certain examples, the rotational axis isvertical. In other examples, the rotational axis may be tilted fromvertical or may be in any orientation that is suitable for theparticular environment in which the sensor assembly 500 is being used.

The chassis 502 has an outer contour that is generally symmetrical aboutthe rotational axis 504. The chassis 502 has a lower section 506(a)having a cylindrical outer contour, an upper section 506(b) having acylindrical outer contour, and a middle section 506(c) having an outercontour that forms a conical frustum between the larger diameter of thelower section 506(a) and the smaller diameter of the upper section506(b).

The upper section 506(b) has a cutout forming a flat surface 508 thatfaces in a forward direction 510, also referred to as the z direction,relative to the chassis 502. The flat surface 508 has one or moreopenings to accommodate first lens 512 and second lens 114. The firstand second lenses 512 and 514 are mounted so that their principal axesare generally perpendicular to the rotational axis 504, and generallyparallel to the forward direction 510. In practice, each of the firstand second lenses 512 and 514 may comprise multiple lenses, such as athree element lens or a “triple lens”, and may therefore have multipleindividual lens elements.

The first and second lenses 512 and 514 have a common field of view of ascene. Rotation of the chassis 502 causes the field of view to move orscan in a scan direction 516, also referred as the x direction, over thescene. In the illustrated example, in which the rotational axis 504 isvertical, the scan direction 516 is horizontal.

The chassis 502 has a partially bisecting internal wall 518 that forms acompartment on each of two lateral sides of the chassis 502. In FIG. 5A,a sensor compartment 520 is shown on one side of the chassis 502. InFIG. 5B, an emitter compartment 522 is shown on the other side of thechassis 502. The sensor compartment 520 houses a light sensor 524. Theemitter compartment houses a laser light source 526.

The first lens 512 is generally above the sensor compartment 520 andforward of the light sensor 524. The second lens 514 is generally abovethe emitter compartment 522 and forward of the laser light source 526.

One or more mirrors 528 are positioned within the chassis 502 behind thefirst and second lenses 512 and 514 to redirect emitted and receivedlight between different directions, such as horizontal and verticaldirections. Received light enters the chassis generally horizontallyfrom the first lens 512 and is redirected downwardly by the one or moremirrors 528 toward the light sensor 524. The laser light source 526emits laser light in an upward direction. The emitted light hits the oneor more mirrors 528 and is redirected horizontally outward, in theforward direction 510 through the second lens 514.

The first lens 512 projects an image onto a sensor frame 530 of thelight sensor 524. The sensor frame 530 is an area having an x axis 534that corresponds optically to the scan direction 516. As the chassis 502rotates, an image of the scene scans along the x axis 534 of the sensorframe 530. Accordingly, the x axis of the sensor frame 530 may at timesbe referred to as the scan axis of the sensor frame 530. In theillustrated orientation in which the rotational axis 504 is vertical,the x axis 534 corresponds optically to the horizontal direction of theprojected image.

The sensor frame 530 has a y axis 536 that is perpendicular to the xaxis. In the illustrated orientation in which the rotational axis 504 isvertical, the y axis 536 of the sensor frame 530 corresponds opticallyto the vertical direction of the projected image.

Laser emitters within an emitter frame 532 of the light source 526project laser light through the second lens 514 into the scene. Theemitter frame 532 has an x axis 538, also referred to as a scan axisthat corresponds optically to the scan direction 516. As the chassis 502rotates, the projected light scans in the scan direction 516. Theemitter frame 532 has ay axis 540 that is perpendicular to the x axis538. In the illustrated orientation in which the rotational axis 504 isvertical, the x axis 538 of the emitter frame 532 corresponds opticallyto the horizontal direction of the scene into which the laser light isprojected. The y axis 540 of the emitter frame 532 corresponds opticallyto the vertical direction of the scene into which the laser light isprojected.

Generally, the laser light source 526 has multiple laser emitters andthe light sensor 524 has multiple corresponding sensor elements. Eachlaser emitter corresponds to a respective sensor element, and a paircomprising an emitter and a corresponding sensor element is referred toas a channel. The term “channel” may also encompass supporting circuitrythat is associated with the emitter/sensor pair. A channel is used toemit a laser light burst and to measure properties of the reflections ofthe burst, as explained below.

While the examples described herein include a plurality of measurementchannels (e.g., 2-100), and accordingly comprise a corresponding numberof laser emitters and respectively corresponding light sensors,different examples may use a single channel or a different number ofchannels depending on desired sensor resolutions and coverage angles,where the coverage angle corresponds to the field of view relative tothe horizon.

FIG. 6 illustrates further details regarding the light sensor 524. Insome configurations, the light sensor 524 comprises an array ofindividual sensor elements 602. In certain examples, the sensor elements602 comprise avalanche photodiodes (APDs).

The sensor elements 602 are mounted on a planar printed circuit board604. The sensor elements 602 are positioned within the sensor frame 530,which is an area within which the first lens 512 projects an image of anexternal scene. FIG. 6 shows the x axis 534, which is the axiscorresponding to the scan direction 516 of the chassis 502 relative tothe scene. The x axis 534, also referred to herein as the scan axis,represents the axis along which an image of the scene is translated asthe chassis 502 rotates.

The sensor elements 602 are arranged in multiple parallel rows, withalternate rows being staggered to achieve a higher packing density. Eachrow extends along a line that is angled with respect to the x axis 534so that each sensor element 602 is at a different elevation relative tothey axis 536, where they axis 536 is orthogonal to the scan axis 534.

FIG. 7 illustrates further details regarding how the sensor elements 602are packed to achieve a relatively high packing density andcorrespondingly fine y-axis pitch. In FIG. 7, an area associated witheach sensor element 602 is illustrated as a hexagon 702, and thehexagons 702 are packed so that they are adjacent to each other. This isknown as hexagonal packing. Each hexagon 702 represents an area that isoccupied by a sensor element 602 and any associated circuitry that maybe located near the sensor element 602.

FIG. 8 illustrates details regarding an example embodiment of the laserlight source 104. The laser light source 104 comprises a plurality ofindividual laser emitters 802, arranged similarly to the sensors asillustrated in FIG. 6. In the described embodiment, the laser emitterscomprise injection laser diodes (ILDs).

The laser emitters 802 are positioned within the emitter frame 532,which is an area from which the lens 514 projects. FIG. 8 shows the xaxis 538, which is the axis corresponding to the scan direction 516 ofthe chassis 502 relative to the scene. In this example, the laseremitters 802 are arranged with the same (or substantially similar)spacing as the sensor elements 702. The laser emitters 802 can bemounted along edges of printed circuit boards, also referred to asemitter boards, with each emitter board being used to position acorresponding row of the laser emitters 802.

FIG. 9 shows an example electrical circuit 900 for driving a laser lightsource. In this example, the circuit 900 provides a single emitted lightpulse. Other circuit configurations, however, can provide multiplepulses. For example, another circuit (not shown) can be configured toprovide two or more emitted light pulses.

The circuit 900 has an inductive boost charging section comprising aninductor 902 and a transistor 904. The transistor 904 may comprise a FETsuch as a GaN FET. A first terminal of the inductor 902 is connected toa power source 906, which has a positive voltage relative to a groundreference 908. For example, the power source 906 may be a 5-volt DC(direct-current) voltage source. The second terminal of the inductor isconnected to the drain of the transistor 904. The source of thetransistor 904 is connected to the ground reference 908.

The electrical circuit 900 has an energy storage capacitor 910. Theenergy storage capacitor 910 is labeled as having a positive (+)terminal and a negative (−) terminal to indicate that during operationof the circuit, the +terminal is charged positively relative to the−terminal.

The energy storage capacitor 910 is connected through a diode 912 to thesecond terminal of the inductor 902, to be charged with current suppliedby the inductor 902. Specifically, the anode of the diode 912 isconnected to the second terminal of the inductor 902. The cathode of thediode 912 is connected to the +terminal of the energy storage capacitor910. The −terminal of the capacitor 910 is connected to the groundreference 908.

The anode of the laser emitter 104 is connected to the +terminal of theenergy storage capacitor 910. A transistor 914 is connected between thecathode of the laser emitter 104 and the ground reference 908.Specifically, the drain of the transistor 914 is connected to thecathode of the laser emitter 104 and the drain of the transistor isconnected to the ground reference 908.

In operation, the gate of the transistor 904 is connected to a chargesignal 916. When the charge signal 916 turns on the transistor 904,current flows from the power source 906, through the inductor 902,through the transistor 904, and to the ground reference 908.

When the current through the inductor 902 is nearly to the saturationpoint of the inductor 902, the transistor 904 is turned off, and theinductor current then flows to the capacitor 910, charging the +terminalrelative to the −terminal.

The gate of the transistor 914 is connected to a trigger signal 918,which is used to turn on the transistor 914 at the appropriate time foremitting a pulse from the laser emitter. Turning on the transistor 914causes the energy stored by the energy storage capacitor 910 todischarge through the laser emitter 104.

The transistor 914 comprises an n-type GaN FET in this embodiment,although a similar circuit may be implemented for use with any FET withappropriate switching capabilities.

As described above, the charging and triggering of the laser emitter canbe at least partially controlled by the controller 112.

FIG. 10A shows an example electrical circuit 1000 for driving anindividual laser emitter 104, and in particular for firing on the laseremitter 104 in a burst of two short pulses. In this example, the laseremitter 104 comprises an injection laser diode having an anode and acathode. Each measurement channel has an instance of the circuit 1000.Note that although the circuit 1000 in this example is configured toproduce two pulses, the circuit 1000 can be expanded to produce anynumber of pulses, and may also be modified to produce only a singlepulse.

The circuit 1000 has an inductive boost charging section comprising aninductor 1002 and a transistor 1004. In certain embodiments, thetransistor 1004 comprises an FET (field-effect transistor) orenhanced-mode GaN FET (gallium nitride field-effect transistor),referred to as an eGaN FET. A first terminal of the inductor 1002 isconnected to a power source 1006, which has a positive voltage relativeto a ground reference 1008. For example, the power source 1006 may be a5-volt DC (direct-current) voltage source. The second terminal of theinductor is connected to the drain of the transistor 1004. The source ofthe transistor 1004 is connected to the ground reference 1008.

The circuit 1000 has first and second energy storage capacitors 1010(a)and 1010(b), which may in some embodiments comprise non-polarizedceramic capacitors. For purposes of discussion, each of these capacitorsis labeled as having an “A” terminal and a “B” terminal. Duringoperation of the circuit, the A terminal is charged positively relativeto the B terminal.

The energy storage capacitors 1010(a) and 1010(b) are connected throughcorresponding blocking diodes 1012(a) and 1012(b) to the second terminalof the inductor 1002, to be charged with current supplied by theinductor 1002. Specifically, the anodes of the blocking diodes 1012(a)and 1012(b) are connected to the second terminal of the inductor 1002.The cathode of the blocking diode 1012(a) is connected to the A terminalof the first energy storage capacitor 1010(a). The cathode of theblocking diode 1012(b) is connected to the A terminal of the secondenergy storage capacitor 1010(b).

The B terminals of the capacitors 1010(a) and 1010(b) are connected incommon to the cathode of the laser emitter 104.

Note that in some cases, the capacitance represented by each of thecapacitors 1010(a) and 1010(b) may be provided by multiple capacitors inparallel.

First and second transistors 1014(a) and 1014(b) are associatedrespectively with the first and second energy storage capacitors 1010(a)and 1010(b). In the described embodiment, each of the transistors1014(a) and 1014(b) comprises an FET, and in some embodiments maycomprise a GaN FET. The drain of the first transistor 1014(a) isconnected to the A terminal of the first energy storage capacitor1010(a). The drain of the second transistor 1014(b) is connected to theA terminal of the second energy storage capacitor 1010(b). The sourcesof the first and second transistors 1014(a) and 1014(b) are connected tothe ground reference 1008. The anode of the laser emitter 104 is alsoconnected to the ground reference 1008.

The circuit 1000 may also have one or more flyback diodes 1016. Theanode of each flyback diode 1016 is connected to the cathode of thelaser emitter 104. The cathode of each flyback diode 1016 is connectedto the anode of the laser emitter 104 and to the ground reference 1008.The flyback diodes limit the negative voltage that can be induced at theanode of the laser emitter 104.

In operation, the gate of the transistor 1004 is connected to a chargesignal 1018. When the charge signal 1018 turns on the transistor 1004,current flows from the power source 1006, through the inductor 1002,through the transistor 1004, and to the ground reference 1008.

When the current through the inductor 1002 is nearly to the saturationpoint of the inductor 1002, the transistor 1004 is turned off, and theinductor current then flows to the capacitors 1010 and positivelycharges the A terminals relative to the B terminals. The relativevoltage to which the capacitors 1010 are charged will be referred toherein as the charge voltage.

In the described embodiments, the transistor 1004 is turned on forapproximately 2 microseconds. When the transistor 1004 is turned off, ittakes approximately 500 nanoseconds for the capacitors 1010 to charge.The total charging time is thus 2.5 microseconds or greater.

The gate of the first transistor 1014(a) is connected to a first triggersignal 1020(a), which is used to turn on the first transistor 1014(a)when the laser emitter 104 is to emit a first pulse. Turning on thefirst transistor 1014(a) lowers the voltage at the A terminal nearly tothe voltage of the ground reference 1008, and accordingly also lowersthe voltage of the B terminal by an amount approximately equal to thecharge voltage. Accordingly, the cathode of the laser emitter 104 willnow be at a negative potential with respect to the anode, and the storedenergy of the capacitor is discharged through the laser emitter 104. Theresulting current through the laser emitter 104 causes the laser emitter104 to emit light.

The gate of the second transistor 1014(b) is connected to a secondtrigger signal 1020(b). The second trigger signal 1020(b) is used todischarge the second capacitor 1010(b) through the laser emitter 104 inorder to create a second pulse.

In operation, the first transistor 1014(a) is turned on to initiate thefirst pulse of a laser burst, and the second transistor 1014(b) isturned on shortly after to initiate the second pulse.

Although the circuit 1000 is shown as using n-type or enhancement modeGaN FETs for the transistors 1014, a similar circuit using p-type ordepletion mode GaN FETs may also be used. In addition, the circuit canbe expanded to support generation of any number of pulses, for use tosequentially fire any number of laser emitters.

In some embodiments, a snubber can be added to reduce voltageoscillations in drive current that might otherwise occur due toparasitic capacitances and inductances. If such oscillations wereallowed to occur, it could become necessary to wait until they were tosubside before firing the laser emitter 104. A snubber may comprise aresistor 1022 and a capacitor 1024 connected between the second terminalof the inductor 1002 and the ground reference 1008 to damp any voltageand current oscillations at the second terminal of the inductor 1002.

The circuit 1000 can be modified to produce any number of laser pulses,including a single pulse or more than two pulses. Dashed lines are usedin FIG. 10A to indicate components of first and second firing circuits1026(a) and 1026(b). These firing circuits can be replicated as neededto create any number of pulses. To create a single drive pulse, a singlefiring circuit 1026 may be used. To create three drive pulses, threefiring circuits 1026 may be used, each connected to the inductor 1002and the emitter 104 as shown in FIG. 10A.

FIG. 10B illustrates additional elements that may be used in someembodiments of a firing circuit 1026 such as shown in FIG. 10A.

Parasitic capacitances and inductances associated with the transistor1014 and its associated components and interconnections may in certainsituations limit the shortness of the pulse generated by the firingcircuit 1026, and it may be desired to produce a shorter pulse thanwould otherwise be possible. In these situations, a relatively smallresistance 1028 may be placed between the A terminal of the energystorage capacitor 1010 and the drain of the transistor 1014. Incombination with parasitic capacitances and inductances, the resistance1028 creates a resonance such that the voltage at the A terminal of thecapacitor 1010 oscillates to produce an initial current pulse that isshorter than would otherwise occur. In some embodiments, a capacitance1030 may also be added between the A terminal of the capacitor 1010 andthe ground reference 1008 to enhance or further tune this effect. Insome embodiments, a capacitance 1032 may similarly be added between theB terminal of the capacitor 1010 and the ground reference 1008 tofurther enhance this effect. The values of the added resistances andcapacitances are calculated or determined based on the characteristicsof the specific implementation in order to achieve a desired initialpulse duration.

In some cases, the transistor 1014 may be duplicated, so that two suchtransistors are used in parallel to drive the current from the energystorage capacitor 1010. Using two transistors in parallel may reduce theeffects of parasitic inductances and capacitances.

Although the discussion above sets forth example implementations of thedescribed techniques, other architectures may be used to implement thedescribed functionality, and are intended to be within the scope of thisdisclosure. Furthermore, although the subject matter has been describedin language specific to structural features and/or methodological acts,it is to be understood that the subject matter defined in the appendedclaims is not necessarily limited to the specific features or actsdescribed. Rather, the specific features and acts are disclosed asexemplary forms of implementing the claims.

What is claimed is:
 1. A system, comprising: an electrical circuitconfigured to produce a drive signal, wherein the drive signal isassociated with a first charge duration; a laser light source coupled tothe electrical circuit, the laser light source configured to receive thedrive signal and emit a first light pulse at a first power level; alight sensor that produces a light signal in response to sensingreflected light corresponding to the first light pulse; a controllercommunicatively coupled to the light sensor, wherein the controller isoperative to: adjust a power of a second light pulse emitted from thelaser light source based at least in part on one or more characteristicsassociated with the light signal, identify a change to one or morevalues of one or more parameters associated with the electrical circuitto change, wherein the change to the one or more values is based atleast on data identified during a calibration routine of the laser lightsource, and cause the laser light source to use the one or more valuesto generate a second light pulse, wherein the second light pulse isemitted at a second power level that is different from the first powerlevel.
 2. The system of claim 1, wherein identify the one or more valuesto change comprises to identify a charging duration based at least inpart on the data identified during the calibration of the laser lightsource, and wherein cause the laser light source to use the one or morevalues comprises to cause the electrical circuit to use the chargingduration to generate the second light pulse.
 3. The system of claim 1,wherein identify the change to the one or more values comprisesaccessing a power profile associated with the laser light source and theelectrical circuit, and retrieving a charging duration based, at leastin part, on a specified power level.
 4. The system of claim 3, whereinthe power profile includes a first charge duration associated with afirst power of a first light pulse, a second charge duration associatedwith a second power of a second light pulse, and a third charge durationassociated with a third power of a third light pulse.
 5. The system ofclaim 1, wherein the electrical circuit further comprises: an inductor;and a capacitor coupled to the inductor, wherein the inductor chargesthe capacitor, and wherein the capacitor is discharged to produce thedrive signal.
 6. The system of claim 1, further comprising ananalog-to-digital-converter (ADC) coupled to the light sensor andconfigured to produce a digitized signal that is a digitalrepresentation of the light signal; and wherein identify the change tothe one or more values includes determining the value based at least inpart on the digital representation of the light signal.
 7. A device,comprising: an electrical circuit including a charging circuit, whereinthe electrical circuit is configured to produce a drive signal based atleast in part on a first charging duration; a laser light source coupledto the electrical circuit, the laser light source configured to receivethe drive signal and emit a first light pulse at a first power level; alight sensor that produces a light signal in response to sensingreflected light corresponding to the first light pulse; a controllercommunicatively coupled to the light sensor, wherein the controller isoperative to identify to change the first charging duration to a secondcharging duration based at least in part on one or more characteristicsof the light signal, determine a value of the second charging durationbased at least in part on calibration data associated with the laserlight source, and cause the laser light source to use the value of thesecond charging duration to emit a second light pulse at a second powerlevel that is different from the first power level.
 8. The device ofclaim 7, wherein cause the laser light source to use the value of thesecond charging time comprises to cause the electrical circuit to chargeone or more capacitors for the value of the second charging time.
 9. Thedevice of claim 7, wherein identify to change the first chargingduration to the second charging duration comprises determining that atleast one of the one or more characteristics of the light signal areoutside of a predefined range, and wherein determine the value of thesecond charging duration is further based at least in part on thepredefined range.
 10. The device of claim 7, wherein determine the valueof the second charging duration comprises accessing a power profileassociated with the laser light source, wherein the power profile isassociated with a calibration of the laser light source.
 11. The deviceof claim 10, wherein the power profile includes a first charge durationassociated with a first power of a first light pulse, a second chargeduration associated with a second power of a second light pulse, and athird charge duration associated with a third power of a third lightpulse.
 12. The device of claim 7, wherein the electrical circuit furthercomprises: an inductor, and a capacitor coupled to the inductor, whereinthe inductor charges the capacitor based at least in part on a chargingtime provided by the controller.
 13. The device of claim 7, furthercomprising: an analog-to-digital-converter (ADC) coupled to the lightsensor and configured to produce a digitized signal that is a digitalrepresentation of the light signal; and wherein determining the value ofthe second charging duration includes determining the value based atleast in part on the digital representation of the light signal.
 14. Thedevice of claim 7, wherein the controller is further operative todetermine a power associated with the light signal, and whereindetermining the value of the second charging duration includesdetermining a power value associated with a desired power range andaccessing the value of the second charging duration within thecalibration data that is associated with the desired power range. 15.The device of claim 7, further comprising: a housing for the laser lightsource, the housing rotatable to optically scan light pulses in a scandirection, the scan direction corresponding to a scan axis of the laserlight source; and wherein the laser light source comprises rows of laseremitters, individual ones of the laser emitters associated with a powerprofile, wherein the power profile includes a first charge durationassociated with a first power of a first light pulse, a second chargeduration associated with a second power of a second light pulse, and athird charge duration associated with a third power of a third lightpulse.
 16. A method, comprising: producing, using an electrical circuitincluding a charge circuit, a drive signal based at least in part on afirst charging duration; emitting, using a laser light source coupled tothe electrical circuit, a first light pulse at a first power level;sensing, using a light sensor, reflected light corresponding to thefirst light pulse; producing a light signal based at least in part onthe reflected light; determining to change the first charging durationbased at least in part on one or more characteristics of the lightsignal; determining a value of a second charging duration based at leastin part on calibration data associated with the laser light source; andcausing the laser light source to use the value of the second chargingduration to emit a second light pulse at a second power level.
 17. Themethod of claim 16, wherein causing the laser light source to use thevalue of the second charging duration comprises causing the chargingcircuit of the electrical circuit to charge one or more capacitors forthe value of the second charging duration.
 18. The method of claim 16,wherein determining the value of the second charging time comprisesaccessing a power profile associated with the laser light source,wherein the power profile is associated with a calibration of the laserlight source.
 19. The method of claim 16, further comprising chargingone or more capacitors of the charging circuit according to the value ofthe second charging duration.
 20. The method of claim 16, furthercomprising producing, using an analog-to-digital-converter (ADC) coupledto the light sensor, a digitized signal that is a digital representationof the light signal, and wherein determining the value of the secondcharging duration includes determining the value based at least in parton the digital representation of the light signal.